Tory leadership front runner Boris Johnson has been warned millions of British expats in Europe are entitled to sue the government if they lose their rights to live and work due to a no-deal Brexit.

Alberto Costa, a senior Conservative back-bencher, issued the warning in a letter to Johnson.

Costa resigned from the government over this issue earlier this year and has tirelessly campaigned for the rights of British expats in Europe as leader of a cross party group in Westminster.

Although Johnson has agreed to safeguard the rights of EU expats if Britain leaves the bloc in a no-deal Brexit, EU chief negotiator Michel Barnier has refused to offer the same terms to British expats.

UK has no power to protect expats

Instead, he told Costa and a team of MPs who travelled to Brussels to meet him over the weekend that the best way to protect the rights of British expats would be for Britain to accept the deal already on the table.

“Whatever the outcome of Brexit, we must protect the rights of the five million citizens affected by Britain’s exit from the European Union,” said Costa after the meeting.

“Mr Barnier reiterated that the best way to do that is through the withdrawal agreement.

“In the event of no-deal the United Kingdom parliament can take measures to protect EU nationals in the UK, but we do not have powers to pass legislation extraterritorially, in other words within the EU, to protect our own citizens.”

Around 1.3 million British expats live in the EU, mostly in Spain, France and the Republic of Ireland.